251 research outputs found
The Presence of Weak Active Galactic Nuclei in High Redshift Star Forming Galaxies
We present [OIII 5007A] observations of the star forming galaxy HDF-BMZ1299
(z=1.598) using Keck Observatory's Adaptive Optics system with the
near-infrared integral field spectrograph OSIRIS. Using previous Halpha and
[NII] measurements of the same source, we are able for the first time to use
spatially resolved observations to place a high-redshift galaxy's substructure
on a traditional HII diagnostic diagram. We find that HDF-BMZ1299's spatially
concentrated nebular ratios in the central ~1.5 kiloparsec (0."2) are best
explained by the presence of an AGN: log([NII]/Halpha)=-0.22+/-0.05 and 2sigma
limit of log([OIII]/Hbeta)>0.26. The dominant energy source of this galaxy is
star formation, and integrating a single aperture across the galaxy yields
nebular ratios that are composite spectra from both AGN and HII regions. The
presence of an embedded AGN in HDF-BMZ1299 may suggest a potential
contamination in a fraction of other high-redshift star forming galaxies, and
we suggest that this may be a source of the "elevated" nebular ratios
previously seen in seeing-limited metallicity studies. HDF-BMZ1299's estimated
AGN luminosity is L_Halpha = 3.7e41 erg/s and L_[OIII] = 5.8e41 erg/s, making
it one of the lowest luminosity AGN discovered at this early epoch.Comment: 15 pages, 4 figures, ApJ Accepted, new version to be published
(updated text, figures, and table
Disturbed eating at high altitude: influence of food preferences, acute mountain sickness and satiation hormones
Purpose: Hypoxia has been shown to reduce energy intake and lead to weight loss, but the underlying mechanisms are unclear. The aim was therefore to assess changes in eating after rapid ascent to 4,559m and to investigate to what extent hypoxia, acute mountain sickness (AMS), food preferences and satiation hormones influence eating behavior. Methods: Participants (n=23) were studied at near sea level (Zurich (ZH), 446m) and on two days after rapid ascent to Capanna Margherita (MG) at 4,559m (MG2 and MG4). Changes in appetite, food preferences and energy intake in an ad libitum meal were assessed. Plasma concentrations of cholecystokinin, peptide tyrosine-tyrosine, gastrin, glucagon and amylin were measured. Peripheral oxygen saturation (SpO2) was monitored, and AMS assessed using the Lake Louis score. Results: Energy intake from the ad libitum meal was reduced on MG2 compared to ZH (643±308 vs. 952±458kcal, p=0.001), but was similar to ZH on MG4 (890±298kcal). Energy intake on all test days was correlated with hunger/satiety scores prior to the meal and AMS scores on MG2 but not with SpO2 on any of the 3days. Liking for high-fat foods before a meal predicted subsequent energy intake on all days. None of the satiation hormones showed significant differences between the 3days. Conclusion: Reduced energy intake after rapid ascent to high altitude is associated with AMS severity. This effect was not directly associated with hypoxia or changes in gastrointestinal hormones. Other peripheral and central factors appear to reduce food intake at high altitud
The Galaxy Structure-Redshift Relationship
There exists a gradual, but persistent, evolutionary effect in the galaxy
population such that galaxy structure and morphology change with redshift. This
galaxy structure-redshift relationship is such that an increasingly large
fraction of all bright and massive galaxies at redshifts 2 < z < 3 are
morphologically peculiar at wavelengths from rest-frame ultraviolet to
rest-frame optical. There are however examples of morphologically selected
spirals and ellipticals at all redshifts up to z ~ 3. At lower redshift, the
bright galaxy population smoothly transforms into normal ellipticals and
spirals. The rate of this transformation strongly depends on redshift, with the
swiftest evolution occurring between 1 < z < 2. This review characterizes the
galaxy structure-redshift relationship, discusses its various physical causes,
and how these are revealing the mechanisms responsible for galaxy formation.Comment: 20 pages, 8 figures. Invited Review to appear in "Penetrating Bars
Through Masks of Cosmic Dust: The Hubble Tuning Fork Strikes A New Note", ed.
D. Block et a
The Host Galaxies of Gamma-Ray Bursts I: ISM Properties of Ten Nearby Long-Duration GRB Hosts
We present the first observations from a large-scale survey of nearby (z < 1)
long-duration gamma-ray burst (LGRB) host galaxies, which consist of eight
rest-frame optical spectra obtained at Keck and Magellan. Along with two host
galaxy observations from the literature, we use optical emission line
diagnostics to determine metallicities, ionization parameters, young stellar
population ages, and star formation rates. We compare the LGRB host
environments to a variety of local and intermediate-redshift galaxy
populations, as well as the newest grid of stellar population synthesis and
photoionization models generated with the Starburst99/Mappings codes. With
these comparisons we investigate whether the GRB host galaxies are consistent
with the properties of the general galaxy population, and therefore whether
they may be used as reliable tracers of star formation. We find that LGRB host
galaxies generally have low-metallicity ISM environments out to z ~ 1. The ISM
properties of our GRB hosts, including metallicity, ionization parameter, and
young stellar population age, are significantly different from the general
galaxy population, host galaxies of nearby broad-lined Type Ic supernovae, and
nearby metal-poor galaxies.Comment: 29 pages, 19 figures, 5 tables, accepted for publication in A
Local Lyman Break Galaxy Analogs: The Impact of Massive Star-forming Clumps on the Interstellar Medium and the Global Structure of Young, Forming Galaxies
We present HST UV/optical imaging, Spitzer mid-IR photometry, and optical
spectroscopy of a sample of 30 low-redshift (z=0.1-0.3) galaxies chosen from
SDSS/GALEX to be accurate local analogs of the high-z Lyman Break Galaxies. The
Lyman Break Analogs (LBAs) are similar in mass, metallicity, dust, SFR, size
and gas velocity dispersion, thus enabling a detailed investigation of
processes that are important at high-z. The optical emission line properties of
LBAs are also similar to those of LBGs, indicating comparable conditions in
their ISM. In the UV, LBAs are characterized by complexes of massive
star-forming "clumps", while in the optical they most often show evidence for
(post-)mergers/interactions. In 6 cases, we find an extremely massive (>10^9
Msun) compact (R~100 pc) dominant central object (DCO). The DCOs are
preferentially found in LBAs with the highest mid-IR luminosities and
correspondingly high SFRs (15-100 Msun/yr). We show that the massive SF clumps
(including the DCOs) have masses much larger than the nuclear super star
clusters seen in normal late type galaxies. However, the DCOs have masses,
sizes, and densities similar to the excess-light/central-cusps seen in typical
elliptical galaxies with masses similar to the LBA galaxies. We suggest that
the DCOs form in present-day examples of the dissipative mergers at high
redshift that are believed to have produced the central-cusps in local
ellipticals. More generally, the properties of the LBAs are consistent with the
idea that instabilities in a gas-rich disk lead to very massive star-forming
clumps that eventually coalesce to form a spheroid. We speculate that the DCOs
are too young at present to be growing a supermassive black hole because they
are still in a supernova-dominated outflow phase.Comment: The Astrophysical Journal, In Press (22 pages, 16 figures). For the
full version with high-resolution colour figures, see:
http://www.mpa-garching.mpg.de/~overzier/Overzier_LBApaper09.pd
An Analytic Model for the Evolution of the Stellar, Gas, and Metal Content of Galaxies
We present an analytic formalism that describes the evolution of the stellar,
gas, and metal content of galaxies. It is based on the idea, inspired by
hydrodynamic simulations, that galaxies live in a slowly-evolving equilibrium
between inflow, outflow, and star formation. We argue that this formalism
broadly captures the behavior of galaxy properties evolving in simulations. The
resulting equilibrium equations for the star formation rate, gas fraction, and
metallicity depend on three key free parameters that represent ejective
feedback, preventive feedback, and re-accretion of ejected material. We
schematically describe how these parameters are constrained by models and
observations. Galaxies perturbed off the equilibrium relations owing to inflow
stochasticity tend to be driven back towards equilibrium, such that deviations
in star formation rate at a given mass are correlated with gas fraction and
anti-correlated with metallicity. After an early gas accumulation epoch,
quiescently star-forming galaxies are expected to be in equilibrium over most
of cosmic time. The equilibrium model provides a simple intuitive framework for
understanding the cosmic evolution of galaxy properties, and centrally features
the cycle of baryons between galaxies and surrounding gas as the driver of
galaxy growth.Comment: 11 pages, MNRAS, accepte
A large sample of low surface brightness disc galaxies from the SDSS- II. Metallicities in surface brightness bins
We study the spectroscopic properties of a large sample of Low Surface
Brightness galaxies (LSBGs) (with B-band central surface brightness mu0(B)>22
mag arcsec^(-2)) selected from the Sloan Digital Sky Survey Data Release 4
(SDSS-DR4) main galaxy sample. A large sample of disk-dominated High Surface
Brightness galaxies (HSBGs, with mu0(B)<22 mag arcsec^(-2)) are also selected
for comparison simultaneously. To study them in more details, these sample
galaxies are further divided into four subgroups according to mu0(B) (in units
of mag arcsec^(-2)): vLSBGs (24.5-22.75),iLSBGs (22.75-22.0), iHSBGs
(22.0-21.25), and vHSBGs (<21.25). The diagnostic diagram from spectral
emission-line ratios shows that the AGN fractions of all the four subgroups are
small (<9%). The 21,032 star-forming galaxies with good quality spectroscopic
observations are further selected for studying their dust extinction,
strong-line ratios, metallicities and stellar mass-metallicities relations. The
vLSBGs have lower extinction values and have less metal-rich and massive
galaxies than the other subgroups. The oxygen abundances of our LSBGs are not
as low as those of the HII regions in LSBGs studied in literature, which could
be because our samples are more luminous, and because of the different
metallicity calibrations used. We find a correlation between 12+log(O/H) and
mu0(B) for vLSBGs, iLSBGs and iHSBGs but show that this could be a result of
correlation between mu0(B) and stellar mass and the well-known mass-metallicity
relation. This large sample shows that LSBGs span a wide range in metallicity
and stellar mass, and they lie nearly on the stellar mass vs. metallicity and
N/O vs. O/H relations of normal galaxies. This suggests that LSBGs and HSBGs
have not had dramatically different star formation and chemical enrichment
histories.Comment: 14 pages, 11 figures, accepted for publication in MNRA
Dissipation and Extra Light in Galactic Nuclei: II. 'Cusp' Ellipticals
We study the origin and properties of 'extra' or 'excess' central light in
the surface brightness profiles of cusp or power-law ellipticals. Dissipational
mergers give rise to two-component profiles: an outer profile established by
violent relaxation acting on stars present in the progenitors prior to the
final merger, and an inner stellar population comprising the extra light,
formed in a compact starburst. Combining a large set of hydrodynamical
simulations with data that span a broad range of profiles and masses, we show
that this picture is borne out -- cusp ellipticals are indeed 'extra light'
ellipticals -- and examine how the properties of this component scale with
global galaxy properties. We show how to robustly separate the 'extra' light,
and demonstrate that observed cusps are reliable tracers of the degree of
dissipation in the spheroid-forming merger. We show that the typical degree of
dissipation is a strong function of stellar mass, tracing observed disk gas
fractions at each mass. We demonstrate a correlation between extra light
content and effective radius at fixed mass: systems with more dissipation are
more compact. The outer shape of the light profile does not depend on mass,
with a mean outer Sersic index ~2.5. We explore how this relates to shapes,
kinematics, and stellar population gradients. Simulations with the gas content
needed to match observed profiles also reproduce observed age, metallicity, and
color gradients, and we show how these can be used as tracers of the degree of
dissipation in spheroid formation.Comment: 40 pages, 32 figures, accepted to ApJ (revised to match accepted
version
"Dark" GRB 080325 in a Dusty Massive Galaxy at z ~ 2
We present optical and near infrared observations of GRB 080325 classified as
a "Dark GRB". Near-infrared observations with Subaru/MOIRCS provided a clear
detection of afterglow in Ks band, although no optical counterpart was
reported. The flux ratio of rest-wavelength optical to X-ray bands of the
afterglow indicates that the dust extinction along the line of sight to the
afterglow is Av = 2.7 - 10 mag. This large extinction is probably the major
reason for optical faintness of GRB 080325. The J - Ks color of the host
galaxy, (J - Ks = 1.3 in AB magnitude), is significantly redder than those for
typical GRB hosts previously identified. In addition to J and Ks bands, optical
images in B, Rc, i', and z' bands with Subaru/Suprime-Cam were obtained at
about one year after the burst, and a photometric redshift of the host is
estimated to be z_{photo} = 1.9. The host luminosity is comparable to L^{*} at
z \sim 2 in contrast to the sub-L^{*} property of typical GRB hosts at lower
redshifts. The best-fit stellar population synthesis model for the host shows
that a large dust extinction (Av = 0.8 mag) attributes to the red nature of the
host and that the host galaxy is massive (M_{*} = 7.0 \times 10^{10} Msun)
which is one of the most massive GRB hosts previously identified. By assuming
that the mass-metallicity relation for star-forming galaxies at z \sim 2 is
applicable for the GRB host, this large stellar mass suggests the high
metallicity environment around GRB 080325, consistent with inferred large
extinction.Comment: 22 pages, 10 figures, accepted for publication in The Astrophysical
Journa
Two-phase galaxy evolution: the cosmic star formation histories of spheroids and discs
From two very simple axioms: (1) that active galactic nucleus activity traces spheroid formation, and (2) that the cosmic star formation history is dominated by spheroid formation at high redshift, we derive simple expressions for the star formation histories of spheroids and discs, and their implied metal enrichment histories.
Adopting a BaldryâGlazebrook initial mass function we use these relations and apply PEGASE.2 to predict the z = 0 cosmic spectral energy distributions (CSEDs) of spheroids and discs. The model predictions compare favourably to the dust-corrected CSED recently reported by the Galaxy And Mass Assembly team from the far-ultraviolet through to the K band. The model also provides a reasonable fit to the total stellar mass contained within spheroid and disc structures as recently reported by the Millennium Galaxy Catalogue team. Three interesting inferences can be made following our axioms: (1) there is a transition redshift at z â 1.7 at which point the Universe switches from what we refer to as âhot mode evolutionâ (i.e. spheroid formation/growth via mergers and/or collapse) to what we term âcold mode evolutionâ (i.e. disc formation/growth via gas infall and minor mergers); (2) there is little or no need for any pre-enrichment prior to the main phase of star formation; (3) in the present Universe mass loss is fairly evenly balanced with star formation holding the integrated stellar mass density close to a constant value.
The model provides a simple prediction of the energy output from spheroid and disc projenitors, the build-up of spheroid and disc mass and the mean metallicity enrichment of the Universe
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